Btrfs: fix ioctl-initiated transactions vs wait_current_trans()

Commit 597:466b27332893 (btrfs_start_transaction: wait for commits in
progress) breaks the transaction start/stop ioctls by making
btrfs_start_transaction conditionally wait for the next transaction to
start.  If an application artificially is holding a transaction open,
things deadlock.

This workaround maintains a count of open ioctl-initiated transactions in
fs_info, and avoids wait_current_trans() if any are currently open (in
start_transaction() and btrfs_throttle()).  The start transaction ioctl
uses a new btrfs_start_ioctl_transaction() that _does_ call
wait_current_trans(), effectively pushing the join/wait decision to the
outer ioctl-initiated transaction.

This more or less neuters btrfs_throttle() when ioctl-initiated
transactions are in use, but that seems like a pretty fundamental
consequence of wrapping lots of write()'s in a transaction.  Btrfs has no
way to tell if the application considers a given operation as part of it's
transaction.

Obviously, if the transaction start/stop ioctls aren't being used, there
is no effect on current behavior.

Signed-off-by: Sage Weil <sage@newdream.net>
---
 ctree.h       |    1 +
 ioctl.c       |   12 +++++++++++-
 transaction.c |   18 +++++++++++++-----
 transaction.h |    2 ++
 4 files changed, 27 insertions(+), 6 deletions(-)

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 11 insertions, 1 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 224da287b3e..0b63c3c77cf 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -715,7 +715,12 @@ long btrfs_ioctl_trans_start(struct file *file)
 		ret = -EINPROGRESS;
 		goto out;
 	}
-	trans = btrfs_start_transaction(root, 0);
+
+	mutex_lock(&root->fs_info->trans_mutex);
+	root->fs_info->open_ioctl_trans++;
+	mutex_unlock(&root->fs_info->trans_mutex);
+
+	trans = btrfs_start_ioctl_transaction(root, 0);
 	if (trans)
 		file->private_data = trans;
 	else
@@ -745,6 +750,11 @@ long btrfs_ioctl_trans_end(struct file *file)
 	}
 	btrfs_end_transaction(trans, root);
 	file->private_data = 0;
+
+	mutex_lock(&root->fs_info->trans_mutex);
+	root->fs_info->open_ioctl_trans--;
+	mutex_unlock(&root->fs_info->trans_mutex);
+
 out:
 	return ret;
 }